Noise produced by the established unsteady flow around aircraft structures, termed airframe noise, is a significant source of aircraft noise during the landing approach phase of flight. Such unsteady flow is prominent around an aircraft's wing structures wherever geometric discontinuities are present, such as gaps, edges, and cavities. In particular, these geometric features are present when leading-edge slat devices and trailing-edge flap devices are deployed to generate high-lift. The leading-edge slats and trailing-edge flaps are extended from the main airfoil element when in the high-lift configuration, thereby feinting structural edges and gaps between the airfoil elements and garnering the terminology multi-element airfoil system. The unsteady flow resulting from the presence of the gap between the leading-edge slat and the main airfoil element is a major source of airframe noise. The unsteady flow resulting from the gap between the main airfoil element and the trailing edge flap is a secondary source of airframe noise.
Conventional transport aircraft wing design is driven mainly by cruise efficiency that is defined by adequate lift generated at cruise speeds for level flight with minimal drag. Wings having high-lift systems (e.g., leading-edge slats and/or trailing-edge flaps) are designed to augment lift and improve stall characteristics at the low speeds required for landing. These multi-element airfoil systems increase the effective chord (i.e., stream-wise dimension) of the wing and thus its effective area. However, the major effect of the multi-element airfoil arrangement is to generate a much larger pressure difference (lift) between the upper (suction) and lower (pressure) surfaces than would be possible via a single airfoil element.
During the cruise phase of flight, the multi-element airfoil system defines a smooth single-element profile to reduce drag. In current practice, the multiple airfoil elements are nested together with the leading edge of each trailing element fitting into a cove in the aft lower surface of the preceding element. When the slat and/or flap elements are deployed, gaps are formed between these elements and the main airfoil element. The unsteady aerodynamic flow resulting from these gaps is the source of considerable noise that radiates outward from the wing. It is highly desirable to develop technologies that reduce this airframe noise without compromising cruise efficiency, or lift and stall characteristics at landing. Additionally, it may be advantageous to control these gaps for other aerodynamic reasons.